1. Field of the Invention
This invention relates to a lubricating oil composition used for internal combustion engines such as gasoline engine and Diesel engine. Particularly it relates to a lubricating oil composition for internal combustion engines which has remarkable effects on both preventing abrasion of valve-moving systems and saving energy, and to a lubricating oil additive used for the composition.
2. Description of the Prior Art
Lubricating oils have recently been required to have increasingly severe performance from the standpoint of saving resources and energy.
As to the lubricating oils for the internal combustion engines, in particular, saving energy type lubricating oils are strongly desired which enhance mechanical efficiency by decreasing friction loss.
Although it is one method for saving fuel cost to reduce viscosity of the lubricating oils, the method is substantially limited.
In order to improve specific fuel consumption by incorporating friction decreasing agents to the lubricating oils and reducing boundary friction, organic molybdenum compounds have generally been used as the friction decreasing agents.
As to such organic molybdenum compounds, primary alkyl molybdenum dithiophosphate prepared by using primary alcohol has been disclosed in, for example, U.S. Pat. Nos. 4,456,538, 4,428,861 and 4,290,902, Japanese Patent Publication Nos. 8426/1965 and 27366/1969, and Japanese Patent Laid-Open Nos. 110796/1981 and 43491/1987. Lubricating oil compositions obtained by the addition of organic molybdenum compounds have been disclosed in Japanese Patent Laid-Open Nos. 39704/1977, 53190/1981 and 122597/1984.
The above organic molybdenum compounds and lubricating oil compositions are found to have considerable effect from the viewpoint of saving energy due to decrease of friction.
The present inventors previously developed a lubricating oil composition in order to improve the specific fuel consumption of the internal combustion engines (Japanese Patent Laid-open No. 240388/1987). The composition contains as requisite components molybdenum dithiophosphate substituted with secondary alkyl groups having 5 or 8 carbon atoms, alkyl zinc dithiophosphate, specific perbasic metal type detergent-dispersant and ashless type detergent-dispersant.
The said lubricating oil composition has a remarkable effect on the decrease in abrasion and improvement in the specific fuel consumption. However, there is a problem that prevention of abrasion in the valve-moving systems, that is, prevention of pitting abrasion at engine cam nose and inhibition of scuffing of rocker pad are dependent upon the selection of alkyl groups of alkyl zinc dithiophosphate.